In many communication systems, such as those employing wideband signals like spread spectrum or very high data rate links, as well as those with stringent constraints in out-of-band spurious levels, the overall channel frequency response results in non-flat in-band amplitude and group delay responses. Such distortions in the frequency response degrade the link performance leading to a requirement for an increased transmit power and/or higher transmit bandwidth.
For example, in a typical wireless transmitter of a wireless communication system, an input signal is provided to a transmitter for transmission therefrom. Typically prior to transmitting the signal, the transmitter conditions the signal so that it is in a form suitable for transmission. Such conditioning typically involves pulse shaping, one or more stages of frequency up-conversion each followed by filtering and amplification before being fed to an antenna, wherein a high-frequency communication signal goes through a number of devices and components, which frequency amplitude and group delay responses may not be flat, resulting in the signal distortion degrading the system performance.
Referring to FIG. 1, a prior art quadrature direct transmitter 10 is shown by way of illustration. The transmitter 10 includes an input port 101, which in operation is fed by an input bit stream of information bits. This input bit stream is received by a mapping circuit 105, that is used to generate an in-phase (I) signal at a first output port and a quadrature (Q) signal at a second output port thereof. Pulse shaping circuit 110 is connected to the output ports of the I/Q mapper, respectively, for receiving the I and Q signals and for pulse shaping thereof. The I and Q pulse shaped signals are converted from a digital domain to an analog domain using digital to analog (DA) converters (DACs) 120. After conversion a vector modulator circuit 125 receives the DA converted signals. Within the vector modulator 125, the analogue I and Q signals independently modulate in-phase and quadrature components of a carrier signal generated by a local oscillator (LO) 128 to produce an up-converted modulated signal, also referred to as the RF signal. Thereafter the RF signal is provided to the RF circuit 130, which may include one or more stages of frequency conversion, filtering and amplification circuits and a power amplifier circuit, and then is fed to an antenna.
The RF circuit 130 may introduce various distortions in the RF signal that degrade the communication system performance. One cause of such distortions is the non-linearity of an amplification characteristic of the power amplifier (PA) that may be included in the RF circuit 130, which introduces non-linear distortions in the amplified signal. These non-linear distortions divert some of the energy from a desired frequency channel into adjacent frequency channels, thereby resulting in a loss of performance within a desired frequency channel as well as the creation of interference within adjacent frequency channels.
U.S. Pat. No. 6,885,241, issued to the inventors of the current invention and assigned to the assignee of the current application, discloses a method and circuit for compensating for these non-linear conversion distortions. According to this method, the amplified signal is sampled at a sampling rate that can advantageously be lower than the data rate of the signal, and statistical information about an envelope function of the amplified signal is collected in the form of a cumulative distribution function (CDF) of the amplified signal's modulation envelope. The method taught in the '241 patent is based on an observation that, when a signal is nonlinearly distorted, the signal envelope CDF is distorted as well. By comparing a reference CDF of the non-distorted signal and the actual CDF at the output of the PA, a pre-distortion function can be derived, which, when applied to the input signal prior to the PA, results in the CDF of the amplified signal that substantially approximates the reference CDF. It was also found that the application of this pre-distortion function to the input signal results in an opening of an eye diagram and a substantial reduction in the bit error rate (BER) of the transmitted signal at the receiver.
Although the method described in the '241 patent has many attractive features and works well in reducing the detrimental effect of the PA non-linearity when such nonlinearities are constant over frequency, it is not directly applicable to linear in-band distortions in the RF circuit 130 where the level of distortion is frequency dependent, such as non-uniform group delay and/or amplitude frequency response of the RF circuit. One reason for this is that the CDF represents a distribution of the signal intensity that is frequency independent, and therefore signal transformations that directly affect the intensity of the modulated signal can be straightforwardly deduced from the CDF's shape. Contrary to that, the linear in-band distortions, rather than change the intensity of the signal, distort its spectrum, and therefore there appears to be no direct and clear way of deducing such distortions from the shape of the CDF. Furthermore, a frequency dependency that is introduced by the in-band group delay and amplitude distortions signify the presence of a memory effect in the circuit, so that the response of the circuit at any given instant of time is dependent of its past. This circuit memory makes impossible the direct mapping of changes in the CDF shape to distortions in the circuit.
Unfortunately, these linear in-band distortions of the transmitted signal become a significant source of errors, especially for broad-band transmission systems operating at high carrier frequencies, e.g. in the wavelength range of a few centimeters or less.
There is a number of prior art methods of compensating for the in-band distortions, such as attempting to design the transmit chain in such a way that it has a frequency response that is flat in amplitude and group delay over the frequency band of interest. However, this approach requires selecting components with tight tolerances, which may be significantly more expensive or may not be available for circuits operating at very high frequencies, such as in the GHz range and above. Often a calibration is required to provide a lookup table that can be then used in operation to control circuit elements over temperature. However, in addition to requiring more complex calibration procedures, such look-up tables are inevitably approximate and their use typically results in insufficient compensation of in-band distortion for some applications.
An object of the present invention is to provide an adaptive distortion compensation circuit and a related method for compensating linear in-band distortions that appear in a transmission path of a communication system.